U.S. patent number 5,895,738 [Application Number 08/916,461] was granted by the patent office on 1999-04-20 for extension of xerocolorgraphy to full color printing employing additive rgb+ k colors.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Gregory J. Kovacs, Delmer G. Parker.
United States Patent |
5,895,738 |
Parker , et al. |
April 20, 1999 |
Extension of xerocolorgraphy to full color printing employing
additive RGB+ K colors
Abstract
An imaging system is provided which combines the perfect
registration capabilities of xerocolography to form perfectly
registered red, green and blue images in a single pass in one mode
of operation. In another mode of operation, the color gamut
possible with RGB toners is extended using black toner to develop
an image that is formed using a second imager or exposure device.
The result is an extended gamut color imaging process using four
colors side by side in a single pass with a minimum amount of color
desaturation and with a minimum number of image registrations. Yet
another mode of operation provides for creating K+2 colors in a
single pass. The three or two colors may be used to form highlight
colors and/or logo colors.
Inventors: |
Parker; Delmer G. (Rochester,
NY), Kovacs; Gregory J. (Mississauga, CA) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
25437306 |
Appl.
No.: |
08/916,461 |
Filed: |
August 22, 1997 |
Current U.S.
Class: |
430/45.31;
399/156; 399/223; 430/54 |
Current CPC
Class: |
H04N
1/502 (20130101); G03G 15/0163 (20130101); G03G
15/0152 (20130101); H04N 1/506 (20130101); G03G
2215/017 (20130101); G03G 2215/0187 (20130101) |
Current International
Class: |
G03G
15/01 (20060101); H04N 1/50 (20060101); G03G
013/01 () |
Field of
Search: |
;430/42,54
;399/223,156 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: RoDee; Christopher D.
Claims
We claim:
1. A method of creating color images during a single pass in a
xerographic printing system using a single exposure, dual
wavelength imaging device and a multilayered charge retentive
structure, said method including the steps of;
uniformly charging said charge retentive structure to a
predetermined voltage level;
using said single exposure, dual wavelength imaging device, forming
spot next to spot CAD and DAD images, a virtual DAD image which is
not immediately developable and background areas, said CAD and DAD
images being at different voltage levels from said each other and
from the voltage level of said background areas and said virtual
DAD image being at the same level as the voltage level of said
background areas;
developing said CAD and DAD images with imaging materials each
containing one pigment of two of the additive primary colors;
conditioning said charge retentive structure to convert said
virtual image to another developable DAD image by flood
illuminating said charge retentive structure with light having a
predetermined wavelength;
developing said another developable DAD image with imaging
materials containing pigment different from said two primary
additive colors;
conditioning a portion of said background area for forming still
another DAD image by uniformly charging the charge retentive
structure to about the same voltage level as said background
areas;
using a second image forming device to form said still another DAD
image in a portion of the undeveloped background area; and
developing said still another DAD image with another imaging
material different from the first three imaging materials.
2. A method of creating color images during a single pass in a
xerographic printing system using additive RGB plus black color
imaging materials, the method comprising the steps of:
(a) providing a multilayered photoreceptor comprising a pair of
photoreceptor structures, each structure comprising a charge
generation layer and a charge transport layer, each structure
responsive to a different light beam wavelength;
(b) providing a single exposure, dual wavelength imaging device
capable of emitting two different light beams, each light beam
having a wavelength for which one of the photoreceptor structures
is responsive;
(c) uniformly charging the photoreceptor to a first predetermined
voltage level;
(d) using the dual wavelength imaging device to expose the charged
photoreceptor and form spot next to spot CAD and DAD images, a
virtual DAD image which is not immediately developable, and
background areas, said CAD and DAD images being at different
voltage levels from each other and different from the voltage level
if said background areas, said virtual DAD image being at the same
voltage level as said background areas;
(e) developing said CAD and DAD images with two of the three RGB
imaging materials;
(f) flood exposing the photoreceptor which an illumination source
emitting light having a wavelength which will reduce the voltage
level of the virtual DAD image to a voltage level which will enable
development by the remaining one of RGB imaging materials;
(g) developing the virtual image with the remaining one of the RGB
imaging materials;
(h) uniformly recharging the photoreceptor to a second
predetermined voltage level equal to the voltage level of the
background areas;
(i) imagewise exposing the background areas of the photoreceptor
with a second imaging device to form another DAD image; and
(j) developing said another DAD image with black imaging material
to complete a composite image of RGB, plus black imaging materials
on the photoreceptor.
3. The method of creating color images as claimed in claim 2,
wherein the first predetermined voltage is V.sub.0, wherein the
second predetermined voltage is V.sub.0 /2; and wherein the
photoreceptor is a belt and said photoreceptor structures are
formed on a conductive support, so that one of the structures is
sandwiched between and in contact with the conductive substrate and
the other of said structures.
4. The method of creating color images as claimed in claim 3,
wherein the dual wave length imaging device is dual wavelength
emitting laser which emits one light beam having red light
wavelength of about 670 nm and a second light beam having an
infrared light wavelength of about 830 nm; and wherein the
photoreceptor structure contacting the conductive substrate is
responsive to infrared light and the other of the two photoreceptor
structures is responsive to red light.
5. The method of creating color images as claimed in claim 4,
wherein said second imaging device is a light beam emitting device
which emits light beams having infrared wavelengths of about 830
nm.
6. The method of creating color images as claimed in claim 5,
wherein the method further comprises: after step (e), recharging
the photoreceptor to the background voltage level.
7. The method of creating color images as claimed in claim 5,
wherein the imaging material used to develop the CAD image is a
positive blue toner; wherein the imaging material used to develop
the DAD image immediately after the CAD image development is
negative green toner; and wherein the imaging material for the
virtual DAD image is negative red toner.
8. The method of creating color images as claimed in claim 5,
wherein the method further comprises:
using a pretransfer corona discharge member to condition the
composite image composed of RGB plus black imaging materials for
transfer to a final substrate;
transferring the composite image to a final substrate;
(m) fusing the composite image to the final substrate by a fuser
assembly to affix the composite image permanently to the final
substrate; and
n) advancing the final substrate with the permanently affixed
composite image to a catch tray for removal therefrom by an
operator.
Description
BACKGROUND OF THE INVENTION
This invention relates to a full color, xerographic printing system
using a Raster Output Scanning (ROS) system incorporating a two
wavelength (.lambda.) laser diode source for the ROS and a charge
retentive surface in the form of a belt or drum structure which is
responsive to the two wavelengths and, more particularly, a red,
green, and blue (RGB) plus black (K) imaging system which minimizes
image desaturation and which can be selectively utilized for
creating perfectly registered RGB process color images or extended
gamut process color using black toner in combination with the RGB
colors or K+2 (black+2 colors) color images where the colors may
comprise highlight and/or logo colors.
Additive p color imaging using RGB color toners requires exact
registration of images. Misregistered images or image overlap of
RGB color toners results in a dirty brown color which desaturates
the colors. This is because RGB colors each absorb two thirds of
the visible spectrum resulting in undesirable browns when overlap
occurs. In contrast, cyan, magenta, and yellow (CMY) colors absorb
only a third of the visible spectrum. Overlaps lead to RGB
colors.
Xerocolography (dry color printing) is a color printing
architecture which combines multi-color xerographic development
with multiwavelength laser diode light sources, with a single
polygon, single optics ROS and with apolychromatic, multilayered
photoreceptor to provide color printing in either a single pass or
in two passes. In a single pass imaging machine, an image is formed
by passing portion of the image receiving member past the
processing stations only once. Inherently perfect registration is
achieved since the various color images are all written at the same
imaging station with the same ROS. In all, three perfectly
registered latent images are written in this manner. Two of the
three images are immediately developable because their voltage
levels are offset from a background level while the voltage level
of the third image is at the time of its creation equal to the
background voltage level. An electrostatically distinguishable
third image is formed when the photoreceptor is exposed to flood
illumination of a predetermined wavelength.
It is desirable to provide as many features in a single imaging
apparatus as possible. One is to create perfectly registered
process color images using the additive primary colors red, green
and blue. It is also desirable to be able to achieve the full gamut
of the RGB images with a minimum of desaturation, and to extend the
gamut further with black or other colors. Another desirable feature
is being able to create K+2 colors where the colors are highlight
colors and/or logo colors.
Following is a discussion of prior art, incorporated herein by
reference, which may bear on the patentability of the present
invention. In addition to possibly having some relevance to the
question of patentability, these references, together with the
detailed description to follow, are intended to provide a better
understanding and appreciation of the present invention.
U.S. Pat. No. 4,731,634 entitled "Apparatus for printing black and
plural highlight color images in a single pass" granted to Howard
M. Stark on Mar. 15, 1988 discloses a method and apparatus for
rendering latent electrostatic images visible using multiple colors
of dry toner or developer and more particularly to printing toner
images in black and at least two highlighting colors in a single
pass of the imaging surface through the processing areas of the
printing apparatus. Two of the toners are attracted to only one
charge level on a charge retentive surface to thereby providing
black and one highlight color while two toners are attracted to
another charge level to form the second highlight color.
U.S. Pat. No. 4,868,611 entitled "Tri-Level Xerography Scorotron
Neutralization Concept" granted to Richard P. Germain on Sep. 19,
1989 discloses the use of a scorotron after the development of a
first image. The scorotron serves to bring that first image to
complete charge neutralization which removes the voltage
responsible for the fringe fields thereby precluding fringe field
development during the development of a subsequent image.
U.S. Pat. No. 5,049,949 entitled "Extension of tri-level xerography
to black plus 2 colors" granted to Parker et al on Sep. 17, 1991
discloses a highlight color printing apparatus and method for
forming one black and two color images. A tri-level image
containing CAD (charged area development) and DAD (discharged area
development) image areas and a background area is formed. A second
DAD image is formed by discharging the background area forming part
of the tri-level image.
U.S. Pat. No. 5,155,541 entitled "Single pass digital printer with
black, white and 2-color capability" granted to Robert P. Loce et
al on Oct. 13, 1992 discloses a method and apparatus for printing
toner images in black and at least two highlighting colors in a
single pass of the imaging surface through the processing areas of
the printing apparatus. Imaging and development techniques of color
photography and tri-level xerography are combined to produce images
with black and two colors wherein the two highlighting colors are
developed with only one color toner. A single imaging step forms a
four level charge pattern on a charge retentive surface followed by
development of two of the image levels using tri-level imaging
techniques. Uniform exposure of the imaging surface, similar to
that used in color photography techniques precedes development of
the last image. The uniform exposure modifies the last developed
image level and the background charge level allowing development of
the last image with a single toner.
U.S. Pat. No. 5,221,954 entitled "Single pass full color printing
system using a quad-level xerographic unit" granted to Ellis D.
Harris on Jun. 22, 1993 discloses a four color toner single pass
color printing system consisting generally of a raster output
scanner (ROS) optical system and a quad-level xerographic unit and
a tri-level xerographic unit in tandem. The resulting color
printing system would be able to produce pixels of black and white
and all six primary colors. The color printing system uses a black
toner and toners of the three subtractive primary colors or just
toners of the three subtractive primary colors.
U.S. Pat. No. 5,223,906 entitled "Four color toner single pass
color printing system using two tri-level xerographic units"
granted to Ellis D. Harris on Jun. 29, 1993 discloses a four color
toner single pass color printing system consisting generally of a
raster output scanner (ROS) optical system and two tri-level
xerographic units in tandem. Only two of the three subtractive
primary colors of cyan, magenta and yellow are available for toner
dot upon toner dot to combine to produce the additive primary
colors. The resulting color printing system would be able to
produce pixels of black and white and five of the six primary
colors, with pixel next to pixel printing producing all but the
strongest saturation of the sixth primary color, an additive
primary color. The color printing system uses either four color
toners or a black toner and three color toners.
U.S. Pat. No. 5,534,990 entitled "Full color printing system using
a penta-level xerographic unit" granted on Jul. 9, 1996 to Ellis D.
Harris discloses a single pass full color printing system
consisting generally of a raster output scanner (ROS) optical
system and a quad-level xerographic unit and a penta-level
xerographic unit in tandem. This full color printing system
produces pixels of black and white and all six primary colors
without toner upon toner.
U.S. Pat. No. 5,337,136 entitled "Tandem Tri-level Process Color
Printer" granted to John F. Knapp et al on Aug. 9, 1994 discloses a
tandem tri-level architecture. Three tri-level engines are arranged
in a tandem configuration. Each engine uses one of the three
primary colors plus one other color. Spot by spot, two color
tri-level images can be created by each of the engines. The spot by
spot images are transferred to an intermediate belt member, either
in a spot on spot manner for forming full color images or in a spot
next to spot manner to form highlight and/or logo color images. The
images created by the tri-level engines can also be transferred to
the intermediate in a manner such that both spot next to spot and
spot on spot transfer is effected.
U.S. Pat. No. 5,347,303 entitled "Full Color Xerographic Printing
System With Dual Wavelength, Single Optical System ROS And Dual
Layer Photoreceptor" granted on Sep. 13, 1994 to Kovacs et al
discloses a full color xerographic printing system, either two pass
or single pass, with a single polygon, single optical system Raster
Output Scanning (ROS) system which has a dual wavelength laser
diode source for the ROS which images the dual beams at a single
station as closely spaced spots or at two stations as closely
spaced spots on a dual layer photoreceptor with each photoreceptor
layer sensitive to or accessible by only one of the two
wavelengths.
U.S. Pat. No. 5,373,313 entitled "Color xerographic printing system
with multiple wavelength, single optical system ROS and multiple
layer photoreceptor" granted to Gregory J. Kovacs on Dec. 13, 1994
discloses single pass color xerographic printing system with a
single polygon, a single optical system Raster Output Scanning
(ROS) system which has a multiple wavelength laser diode source for
the ROS which images the multiple beams at a single station as
closely spaced spots on a multiple layer photoreceptor with each
photoreceptor layer sensitive to or accessible to only one of the
multiple wavelengths.
U.S. Pat. No. 5,444,463 entitled "Color xerographic printing system
with dual wavelength, single optical system ROS and dual layer
photoreceptor" granted to Kovacs et al on Aug. 22, 1995 discloses a
single pass color xerographic printing system with a single
polygon, single optical system Raster Output Scanning (ROS) system
which has a dual wavelength laser diode source for the ROS which
images the dual beams at a single station as closely spaced spots
on a dual layer photoreceptor with each photoreceptor layer
sensitive to or accessible by only one of the two wavelengths.
U.S. Pat. No. 5,565,974 entitled "Penta-level xerographic unit"
granted to Ellis D. Harris on Oct. 15, 1996 discloses a penta-level
xerographic unit which produces five exposure levels on a
photoreceptor. The five exposure levels select between a
subtractive and an adjacent additive primary color in both the CAD
and DAD operational regimes of a xerographic process. Exposure
levels intermediate between the CAD and the DAD result in white.
The selection of two possible colors in CAD, or two possible colors
in DAD, or the selection of no toner yields a possibility of five
colors. This penta-level xerographic unit can be used for a K+3
reduced color gamut printer, typically cyan, yellow and red plus
black.
U.S. Pat. No. 5,592,281 entitled "Development scheme for three
color highlight color trilevel xerography" granted to Parker et al
on Jan. 7, 1997 discloses the creation of multiple color images in
a single pass utilizing a multilayered photoreceptor structure
having layers which are responsive to different wavelength lasers.
A composite image including three images areas is formed with
substantially perfect registration. A CAD and DAD image are
developed using CMB (conductive magnetic brush) development and a
second DAD image is developed using a non-interactive development
system. Development of the second DAD image without developing
halos around the CAD image is accomplished by uniformly recharging
the photoreceptor to the background potential prior to the
formation and development of the second DAD image.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the present invention, an imaging system is
provided which combines the perfect registration capabilities of
xerocolography to form perfectly registered red, green and blue
images in a single pass in one mode of operation.
In another mode of operation, the color gamut possible using RGB
toners is extended using black or another color toner to develop an
image that is formed using a second imager or exposure device. The
result is an extended gamut color imaging process using four colors
side by side in a single pass with a minimum amount of color
desaturation and with a minimum number of image registrations.
Yet another mode of operation provides for creating K+2 colors in a
single pass. The two colors may be used to form highlight colors
and/or logo colors.
The various modes of operation are made possible because some of
the imaging components are selectively actuatable via a user
interface.
The three perfectly registered images are developed using the
additive primary colors RGB to create color images. Extended gamut
RGBK composite images are made possible by creating the black
component using a second exposure and development step. This can be
done in a single pass using a second exposure station or in two
pass imaging with only one exposure station. The K+RGB combination
gives a desirable color gamut and the single pass process gives
high throughput. The color gamut provided by the disclosed imaging
system is similar to the color gamut available on an RGB type CRT
(cathode ray tube) display. The perfect registration inherent in 3
color, 2 wavelength xerocolography is somewhat compromised by the
introduction of the second imager but the additional black image
capability and resulting broader color gamut outweigh this
disadvantage in many applications.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a dual layer, dual wavelength
photoreceptor belt for use in the xerographic printing system of
FIG. 8.
FIG. 2 is a schematic illustration of the state of the
photoreceptor following initial exposure thereof.
FIG. 3 depicts a xerocolographic, latent, tri-level image
profile.
FIG. 4 depicts color development steps using a two color or two
layer photoreceptor and RGB pigments.
FIG. 5 depicts color development steps using a two color or two
layer photoreceptor and RGB pigments together with an additional
step of creating a black image.
FIGS. 6A and 6B shows a comparison of the color gamuts which can be
achieved with the RGB and CMY color schemes.
FIG. 7 shows the colors achievable with RGB additive colors
compared to those achievable with CMY subtractive colors.
FIG. 8 is a schematic illustration of the imaging process according
to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE
INVENTION
As shown in FIG. 8, xerocolography engine 8 comprises a charge
retentive member in the form of a photoconductive belt structure 10
comprising a photoconductive surface and an electrically conductive
substrate. The belt 10 is mounted for movement past a charging
station A, a first image exposure station B, a first development
station C, a second development station D, a recharge station 90, a
flood illumination station E, a third development station F, a
second recharge station G, a second exposure station H, a fourth
development station I, a pretransfer charging station J and a
transfer station K.
Belt 10 moves in the direction of arrow 12 to advance successive
portions thereof sequentially through various processing stations
disposed about the path of movement thereof. Belt 10 is entrained
about a plurality of rollers 14 and 16. The roller 16 may be used
as a drive roller and the roller 14 may be used to provide suitable
tensioning of the photoreceptor belt 10. Motor 20 rotates roller 16
to advance belt 10 in the direction of arrow 12. Roller 16 is
coupled to motor 20 by suitable means, not shown.
As can be seen by further reference to FIG. 8, initially successive
portions of belt 10 pass through charging station A. At charging
station A, a corona discharge device such as a scorotron, corotron
or dicorotron indicated generally by the reference numeral 22,
charges the belt 10 to a selectively high uniform positive or
negative potential.
Next, the uniformly charged portions of the photoreceptor surface
are advanced through exposure station B. At exposure station B, the
uniformly charged photoreceptor or charge retentive surface 10 is
exposed to a laser based Raster Output Scanning (ROS) device 24
which effects selective discharge of the photoreceptor belt
structure 10. Any suitable control such as an Electronic SubSystem
(ESS) 23, well known in the art, may be employed for controlling
the ROS modulation device 24 as well as controlling the functions
of the engine 8.
The ROS 24 can use a dual wavelength hybrid or monolithically
integrated laser semiconductor structure 26 consisting of a red,
e.g. 670 nm, wavelength laser emitter such as a semiconductor
structure of AlGaInP/GaAs and an infrared, e.g. 830 nm, laser
emitter such as a semiconductor structure of AlGaAs/GaAs, both
laser emitter structures being known to those of ordinary skill in
the art.
The different wavelength beams 30 and 32 are scanned sequentially
over each other on the photoreceptor to yield excellent
registration . The tangential offset of the laser sources is given
an upper limit of 300 .mu.m since tangential offset does not
introduce scan line bow. The effect of tangential offset is to
require delay in the electronic modulation signals to one of the
dual beams relative to the other since one beam lags the other
during scanning across the photoreceptor. Sagittal offset can also
be used so that the beams are simultaneously scanning adjacent
lines. On each successive scan, the line previously scanned by the
forward beam is overscanned by the trailing beam. Appropriate image
processing algorithms produce the desired image. The dual
wavelength laser structure provides a substantially common spatial
origin for each beam. Each beam is independently modulated so that
it exposes the photoreceptor structure in accordance with a
respective color image.
The two laser beams 30 and 32 emitted from the laser structure 26
are directed to a conventional beam input optical system 40 which
collimates, conditions and focuses the beams onto optical paths
such that they impinge on a rotating polygon mirror 42 having a
plurality of facets 44. As the polygon mirror rotates, the facets
cause the reflected beams to deflect repeatedly in the direction
indicated by the arrow 46. The deflected laser beams are input to a
single set of imaging and correction optics 48, which corrects for
errors such as polygon angle error and wobble and focuses the beams
onto the photoreceptor belt structure 10.
As illustrated in FIG. 1 the photoreceptor belt 10 consists of a
flexible electrically conductive substrate 50. The substrate can be
opaque, translucent, semi-transparent, or transparent, and can be
of any suitable conductive material, including copper, brass,
nickel, zinc, chromium, stainless steel, conductive plastics and
rubbers, aluminum, semitransparent aluminum, steel, cadmium,
silver, gold, paper rendered conductive by the inclusion of a
suitable material therein or through conditioning in a humid
atmosphere to ensure the presence of sufficient water content to
render the material conductive, indium, tin, metal oxides,
including tin oxide and indium tin oxide, and the like. In
addition, the substrate can comprise an insulative layer with a
conductive coating, such as vacuum-deposited metallization on
plastic, such as titanized or aluminized Mylar.TM. polyester,
wherein the metalized surface is in contact with the bottom
photoreceptor layer or any other layer such as a charge injection
blocking or adhesive layer situated between the substrate and the
bottom photoreceptor layer. The substrate has any effective
thickness, typically from about 6 to about 250 microns, and
preferably from about 50 to about 200 microns, although the
thickness can be outside of this range. The photoreceptor belt
further comprises a pair of photoreceptor structures each including
a charge generation layer and a charge transport layer.
Adhered to the substrate 50 is a first or lower generator layer 52
of GaOHPc approximately 0.1 to 1 .mu.m thick, a first or lower
transport layer 54 of
N,N'-diphenyl-N,N'-bis(3"-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine
(TPD) in polycarbonate which is hole transporting and approximately
15 .mu.m thick, a second or upper generator layer 56 of
benzimidazole perylene (BZP) approximately 0.1 to 1 .mu.m thick, a
second or upper transport layer 58 of TPD in polycarbonate which is
hole transporting and approximately 15 .mu.m thick.
The GaOHPc generator layer is thin enough to maintain low dark
decay and the BZP generator layer is thick enough to be opaque to
the wavelength used to discharge it. BZP is known to be coatable to
opaque thicknesses while maintaining low dark decay.
For this illustrative example, the GaOHPc generator layer is
infrared sensitive at 830 nm and the BZP generator layer is red
sensitive at 670 nm. Each generator layer can only be accessed by
one of the two wavelengths. The BZP layer does not absorb the 830
nm wavelength and passes it to the GaOHPc layer. The 670 nm
wavelength is absorbed by the BZP layer and is not transmitted to
the GaOHPc layer (which would also be sensitive to the 670 nm
light).
The generator and transport layers can be deposited by vacuum
evaporation or solvent coating upon the substrate by means known to
those of ordinary skill in the art.
During exposure of the photoreceptor belt 10 to the light beams
from the ROS 24, the 670 nm wavelength of one modulated beam would
be entirely absorbed in the opaque BZP generator layer. Exposure
with the 670 nm beam would therefore discharge the BZP and upper
transport layer 58. None of the 670 nm light beam would reach the
GaOHPc layer so that it and the lower transport layer 54 would
remain fully charged. The second wavelength is chosen to be 830 nm
to insure that it will pass completely through the BZP layer
without effecting any discharge of that layer or upper transport
layer 58. However, the GaOHPc layer is sensitive to 830 nm and
exposure with this wavelength from a modulated beam will discharge
that layer and the lower transport layer 54. The 830 nm exposure
should not be allowed to effect discharge through the benzimidazole
perylene layer and the upper transport layer.
As illustrated in FIG. 2, exposure of an area of the photoreceptor
belt 10 to the both wavelengths or to only one of the wavelengths
results in the photoreceptor being electrostatically conditioned as
follows: (a) the unexposed areas which retain the original surface
voltage, (b) areas exposed with the 830 nm beam which are
discharged to roughly one-half of the original surface voltage, (c)
areas exposed with the 670 nm beam which are also discharged to
roughly one-half of the original photoreceptor voltage, V.sub.0 and
(d) the areas exposed with both the 830 nm and 670 nm wavelength
beams which are fully discharged. While only three voltage levels
are present on the photoreceptor immediately following exposure,
there will be four distinctly different areas after xerographic
development during the first pass of the photoreceptor through the
process stations. While the surface voltages in regions (b) and (c)
are roughly equal after exposure they have been formed in very
distinct ways. During the development process the photoreceptor
will remember how these voltages were formed to allow development
in very different ways in the two regions.
The image area represented by (a) corresponds to the CAD portion of
a tri-level image while the image area represented by (d)
corresponds to the DAD portion of a tri-level image. The areas
represented by (b) and (c) in FIG. 2 are at a voltage level
corresponding to the background level of the tri-level image.
Because of the way these images were formed the area (b) represents
a second DAD image area which initially is not distinguishable from
the background voltage level at (c). At the appropriate point in
the imaging process, the second DAD image is rendered
distinguishable so that it can be developed.
As shown in FIG. 3, the photoreceptor voltage profile of the
photoreceptor 10 after exposure is such that it contains a
tri-level image 60 comprising a charged image area 62, a discharged
image area 64 and a background area 66. The tri-level image also
includes initially a virtual image 68 which is at the same voltage
level as the background voltage.
As illustrated in FIG. 8, as the negatively charged photoreceptor
belt moves past a blue developer housing structure 80 where the CAD
image 62 is developed with positively charged blue toner deposited
thereon via a donor roll structure 82. While the developer housing
structure is illustrated as being a Non Interactive Development
(NID) device a magnetic brush development system may also be
employed since the CAD image is the first image developed.
As the tri-level image is moved past a green developer housing
structure 84 negatively charged green toner is deposited on the DAD
image area 64 via an electroded donor roll structure 86. This
development station could also employ a soft magnetic brush
development system.
The order of the CAD and first DAD development can be reversed
since there are no intervening steps in the process.
Following development of the CAD and DAD images, the photoreceptor
is uniformly recharged to the background level 66 using a corona
discharge device 90 such as a scorotron or dicorotron. The recharge
step is followed by flood exposing the entire photoreceptor 10
using an illumination source 92 operating at a suitable wavelength.
The effect of this exposure step is to discharge regions of the
photoreceptor containing the virtual image 68 thereby forming a
second developable DAD image.
The toners used to develop the CAD and first DAD images are opaque
to light at the wavelength of the flood exposure in order to avoid
developing a voltage offset after the recharge and flood
exposure.
The second developable DAD image is developed with red negatively
charged toner using an NID device 93 including an electroded donor
roll 94.
The photoreceptor 10 is recharged using a corona discharge device
100. This recharge step is followed by an imagewise exposure using
an infrared wavelength, e.g. 830 nm, ROS or Light Emitting Diode
(LED) array 102. The device 102 imagewise exposes a portion of the
white (undeveloped) background area 66 of the photoreceptor. The
image formed in this manner is subsequently developed using a DAD
developer system 104 including an electroded donor roll 105 which
deposits negatively charged black toner on the image.
Each of the components of the imaging apparatus are selectively
actuatable though the control the an Electronic SubSystem (ESS) 23
and a User Interface (UI) 106. Thus, pursuant to the invention and
in addition to being able to develop perfectly registered RGB
images together with black images as discussed above, K+2 colors
can also be achieved. One way for K+2 colors to be accomplished, is
for the red developer device 93, recharge device 100 and imagewise
exposure device 102 to be rendered inoperative in response to a
program selected by an operator using the UI 106. In this case the
colors as well as the black are perfectly registered because the
images are all written simultaneously with the same ROS at the same
imaging station. All other K+2 combinations require recharge 100
and exposure 102 be operative. In these cases only the colors are
perfectly registered.
In another mode of operation, a narrower color gamut imaging with
perfectly registered RGB images is also possible. For this purpose
the recharge device 100, imagewise exposure device 102 and the
black developer unit 104 are rendered inoperative.
The electrostatics involved using RGB toners are depicted in FIG.
4. As shown therein, a tri-level image 107 is formed as the result
of the imagewise exposure, step 1. Normally a photoreceptor is
discharged to a small but non zero potential. For simplicity this
residual voltage after full exposure is taken to be zero volts in
FIG. 4. Furthermore it is assumed in FIG. 4 that the charge on the
developed toner brings the image potential to the same level as the
development bias. Step 2 involves developing the CAD image using
blue toner where the blue developer structure is electrically
biased at about 100 volts offset from the background voltage
effecting deposition of positively charged blue toner on the CAD
image. The rows of the table of FIG. 4 show the charge level of the
CAD, DAD and background areas of the photoreceptor at any given
step in the process as well as the color toner developed on a given
image area. Step 3 of the process effects development of the DAD
image with green toner with the green developer housing bias being
the offset from the background by about 100 volts in the opposite
direction from the bias for the blue housing. Following development
of the CAD and DAD images, the photoreceptor is recharged to mid
level (step 4) and flood illuminated (step 5) in order to form the
second DAD image at 0 volts (ideal situation). The DAD image
created by the flood illumination step is then developed with
negatively charged red toner, step 6. The bias for this development
step is offset by about 100 volts from the midlevel which is the
same as the offset used for the other DAD housing.
The electrostatics involved using RGBK toners are depicted in FIG.
5. The process steps for RGBK are identical to steps 1 through 6
for the RGB process illustrated in FIG. 4. For RGBK, steps 7, 8 and
9 are added. In step 7 the photoreceptor is uniformly charged to
V.sub.0 /2 (assumed here to be 400 V). Step 8 shows the imagewise
exposure with infrared light, e.g. 830 nm, of a portion of the
nondeveloped background area which creates a DAD image at residual
potential, taken to be 0 volts in this case. Step 9 provides for
development of the image created using the imagewise exposure
device 102 as shown in FIG. 8. This third DAD development is done
with black toner with the black developer housing bias being set at
about 100 volts offset from the mid level.
Because the composite image developed on the photoreceptor consists
of both positive and negative toner, a pretransfer corona discharge
member 112 disposed at pretransfer charging station J is provided
to condition the toner for effective transfer to a substrate using
positive corona discharge. The pretransfer corona discharge member
is preferably an AC corona device biased with a DC voltage to
operate in a field sensitive mode and to perform tri-level
xerography pretransfer charging in a way that selectively adds more
charge (or at least comparable charge) to the part of composite
tri-level image that must have its polarity reversed compared to
elsewhere. This charge discrimination can be enhanced by
discharging the photoreceptor carrying the composite developed
latent image with light (not shown) before the pretransfer charging
begins. Furthermore, flooding the photoreceptor with light
coincident with the pretransfer charging minimizes the tendency to
overcharge portions of the image which are already at the correct
polarity.
The colors achievable with RGB additive colors compared to those
achievable with CMY subtractive colors are shown in FIGS. 6A and
6B. In these figures the vertical axes are designated as A for
absorption, R for reflection and T for transmission of light. The
horizontal axes show the wavelength of light and its division into
spectral components. In general, the CMY colors are not saturated
in the RGB scheme. For example, the reflection of G and R light
from Y toner on paper is very high in the color-on-color CMY
scheme. The equivalent reflection of G and R light from the
color-beside-color RGB scheme is only about half the value since
the R toner creates unwanted G absorption and the G toner creates
unwanted R absorption. In the color-beside-color RGB scheme, the
CMY colors are not saturated but the RGB colors are saturated. In
the color-on-color CMY scheme the CMY colors are saturated but the
RGB colors are not fully saturated due to the nature of the color
pigment absorptions.
In FIG. 7 a comparison of the color gamuts which can be achieved
with the two schemes is shown in the a*b* plane of the CIELab color
coordinate system. The total gamut of the CMY scheme is generally
better overall but lacks the saturation of the RGB colors
obtainable with the RGB pigments themselves. The saturation of the
CMY colors is of course better in the subtractive scheme because of
the presence of the CMY pigments themselves. However, the
color-beside-color RGB scheme still gives a good color gamut which
is similar to that of a CRT and is adequate for many
applications.
As shown in FIG. 7, the color gamut which can be achieved with the
color on color (CMY) scheme is the area enclosed by the solid line.
The color gamut which can be achieved with the color beside color
(RGB) scheme is the area enclosed by the dashed line. In the
example, the RGB colors in the additive scheme will generally have
greater saturation than in the subtractive scheme due to the nature
of the RGB color pigments. The color on color CMY pigments do not
generally give as saturated RGB colors as the RGB pigments
themselves.
The images created using the green, blue, red and black toners, as
shown in FIG. 8, are transferred to a final substrate 114, such
transfer taking place at transfer station K.
Transfer station K includes a corona generating device 116 which
sprays ions of a suitable polarity onto the backside of substrate
114. This attracts the charged toner powder images from the
photoreceptor belt 10 to the substrate.
After the images have been transferred to the substrate 114 from
photoconductive surface of belt 10, the residual toner particles
carried by the photoconductive surface are removed therefrom. These
particles are removed at cleaning station L. A magnetic brush
cleaner housing is disposed at the cleaner station L. The cleaner
apparatus comprises a conventional magnetic brush roll structure
118 for causing carrier particles in the cleaner housing to form a
brush-like orientation relative to the roll structure and the
charge retentive surface. It may also include a pair of detoning
rolls (not shown) for removing the residual toner from the brush.
Other cleaning systems, such as fur brush or blade, are also
suitable.
Subsequent to cleaning, a discharge lamp 113 positioned at station
M floods the photoconductive surface with white light to dissipate
any residual electrostatic charge remaining prior to the charging
thereof for the successive imaging cycle.
Fusing station N includes a fuser assembly, indicated generally by
the reference numeral 134, which permanently affixes the
transferred powder image to substrate 114. Preferably, fuser
assembly 134 comprises a heated fuser roller 136 and a backup
roller 138. Substrate 114 passes between fuser roller 136 and
backup roller 138 with the toner powder images contacting fuser
roller 136. In this manner, the toner powder image is permanently
affixed to substrate 114. After fusing, a chute, not shown, guides
the advancing substrate 114 to a catch tray, also not shown, for
subsequent removal from the printing machine by the operator.
* * * * *